1. Field of the Invention
The present invention relates to an organic EL panel having a plurality of organic EL elements arranged in a matrix.
2. Description of the Related Art
Organic EL display panels are one type of conventionally known flat display panel. Organic EL display panels differ from liquid crystal display panels (LCDs) in that they are self-emissive, and are very much expected to come into wide use as bright and easy-to-view flat display panels.
A typical organic EL display comprises a number of organic EL elements as pixels arranged in a matrix. Organic EL elements, similar to the elements in LCDs, can be driven using either a passive or active method. Also similar to LCD elements, an active matrix method, in which a switch element is provided for each pixel and turning on and off of each switch element is controlled to thereby control display of each element, is preferred because a screen image with more finer detail can be achieved with an active matrix method, rather than a passive method in which a switch element is not provided for each pixel.
FIG. 4 shows an example structure of a pixel circuit in an organic EL panel which utilizes a conventional thin film transistor (TFT). The organic EL panel is configured by arranging these pixels in a matrix.
A gate line GL, which runs in the row direction, is connected to the gate of a first TFT 10, which is an N-channel TFT for selection by the gate line GL. The first TFT 10 is further connected at its drain to a data line DL, which runs in the column direction, and at its source to a storage capacitor CS. The storage capacitor CS is connected at its other end to a capacitor line SL. The joint between the source of the first TFT 10 and the capacitor CS is connected to the gate of a second TFT 21, which is a p-channel TFT. The second TFT 21 is further connected at its source to a power source line VL and at its drain to an organic EL element 50. The organic EL element 50 is further connected at its other end to a cathode source VC.
A gate line GL is provided for each horizontal line of pixels, and is configured to be set at a high level by the vertical drive circuit 60. That is, the respective gate lines GL are sequentially configured to be set at an H level, to thereby turn on all of the first TFTs 10 which are connected at their gates to these gate lines GL set at an H level.
Meanwhile, the data line DL is connected to a horizontal drive circuit 62 which couples the video signal line and the data line DL at timing when a corresponding displaying video signal (data) is supplied to the video signal line.
As described above, the first TFT 10 is turned on in response to a gate line GL is set at an H level, and data on the data line DL is held in the storage capacitor CS. Subsequently, the second TFT 21 is turned on according to the data (potential) held in the storage capacitor CS, and, while the second TFT 21 remains in an ON state, a current is caused to flow into the organic EL element EL for light emission.
As shown in FIG. 5, the vertical drive circuit 60 for controlling the gate line GL and the horizontal drive circuit 62 for controlling the data line DL are provided near the periphery of the display area where organic EL elements are arranged in a matrix. A power source and various signals which are necessary for the organic EL panel are supplied via an interface 64, which is also provided near the periphery of the display area.
Here, the second TFT 21 must be able to provide stable supply of a current over a wide range according to the voltage held in the capacitor. In addition, minimization of the required current and of power consumption are desired.
In order to satisfy these requirements, one solution may be to set a low power source voltage for the second TFT 21. Although this can reduce the current required in the second TFT while maintaining a range of adjustment for the current, lowpower voltage for the second TFT 21 may also cause a problem of unsuccessful control of the amount of emitted light due to variations in characteristics of the organic EL elements and the second TFTs 21.